Does a Retaining Wall Need Drainage?
Most retaining wall failures are not a failure of the wall itself. They are a failure to deal with water. Soil behind a wall holds water, and water has weight and pressure. Left to build up, it loads the wall far beyond what the wall was built to resist. This is why drainage is not an optional extra on a retaining wall, it is part of the structure. This guide explains why, what good drainage looks like, and what happens when it is left out.
Why Does a Retaining Wall Need Drainage?
A retaining wall needs drainage because water trapped in the soil behind it creates hydrostatic pressure that adds to the earth pressure the wall already carries. As the water table behind the wall rises, that pressure increases quickly and can double the total load pushing the wall outward. Drainage relieves the pressure by giving the water a path out before it can build up, so the wall only has to resist the soil, not the soil plus a column of water.
What Drainage Should a Retaining Wall Have?
A retaining wall should have a free draining zone behind it and a subsoil drain at the base, both discharging to a legal point. A structural engineer specifies these as part of the design to AS 4678, the Australian Standard for earth retaining structures. The table below sets out the usual components.
| Component | What it does |
|---|---|
| Free draining backfill | Coarse aggregate directly behind the wall that lets water fall to the base instead of saturating the soil. |
| Geotextile filter fabric | Separates the aggregate from the retained soil so fine material does not wash in and clog the drainage. |
| Subsoil (agricultural) drain | A perforated pipe at the base of the wall that collects the water and carries it away. |
| Weep holes (some walls) | Openings through the face of a masonry or concrete wall that let water escape at the base. |
| Outlet to a legal point of discharge | Where the collected water is directed, so it leaves the site correctly rather than pooling. |
The principle behind all of it is simple. Give the water somewhere to go before it can sit against the wall. The aggregate lets it fall, the fabric keeps the drainage clear, and the subsoil drain takes it away.
What Happens if a Retaining Wall Has No Drainage?
If a retaining wall has no drainage, water builds up in the soil behind it and the pressure on the wall rises until the wall moves. The first signs are usually the wall leaning or bulging, cracking through the wall or its footing, and water seeping or staining at the base. Left unaddressed the wall can slide, overturn, or fail, and a failure can damage neighbouring property, driveways, and buildings. Because water pressure can double the load, a wall built without drainage is loaded well beyond what it was designed to carry.
How Deep Should Drainage Be Behind a Retaining Wall?
The drainage should run the full height of the retained soil, with the subsoil drain sitting at the base of the wall behind the footing. The free draining aggregate zone extends up behind the wall so water at any level can fall to the drain rather than being trapped higher up. A structural engineer sets the exact arrangement for the specific wall, because it depends on the wall height, the soil, and any surcharge from driveways or buildings above.
Frequently asked questions
Does a retaining wall need drainage?
Yes. Water building up in the soil behind a retaining wall creates hydrostatic pressure that can double the load on the wall and push it over. Drainage removes the water before it can load the wall. A structural engineer designs the drainage as part of the wall to AS 4678, typically a free draining aggregate zone and a subsoil drain discharging to a legal point.
What is the best drainage for a retaining wall?
The best drainage is a free draining aggregate zone directly behind the wall, separated from the retained soil by a geotextile filter fabric, with a perforated subsoil drain at the base carrying water to a legal point of discharge. Some masonry and concrete walls also use weep holes. A structural engineer specifies the arrangement for the wall height, the soil, and any surcharge.
What happens if a retaining wall has no drainage?
Water builds up behind the wall and the pressure rises until the wall moves. You may see the wall leaning or bulging, cracking through the wall or footing, and water seeping at the base, and in severe cases the wall can slide or overturn and fail. Because water pressure can double the load, a wall without drainage is loaded well beyond its design.
How deep should drainage be behind a retaining wall?
The drainage should run the full height of the retained soil, with the subsoil drain at the base of the wall behind the footing and the free draining aggregate zone extending up behind the wall. A structural engineer sets the exact arrangement, because it depends on the wall height, the soil, and any surcharge from driveways or buildings above.
What Australian Standard covers retaining wall drainage?
Earth retaining structures in Australia are designed to AS 4678, which a structural engineer applies to the wall, the footing, the reinforcement, and the drainage together. The drainage is designed as part of the wall, not added afterwards, because relieving water pressure is central to how the wall performs.
Related PBE services. See retaining wall design for the full engineering service, and structural engineer inspections if an existing wall is leaning or showing movement.
Designing or repairing a retaining wall in Melbourne?
Principal Built Engineering designs retaining walls and their drainage to Australian Standards for builders, developers, and owners across Melbourne and Victoria, with documentation delivered in approximately two weeks.
Get in touchLast reviewed: July 2026 by Principal Built Engineering, structural engineers, Melbourne.
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